Many small printers use a battery as the main power supply and the output voltage of the battery is lowered as the power is consumed in such known printers. When the output voltage of the battery falls below a predetermined value, the printer may often operate incorrectly. Therefore, it is necessary to detect when the output voltage of the battery falls below a predetermined value so that the battery can replaced or recharged. A conventional circuit for detecting when the battery voltage falls below a predetermined minimum is shown in FIG. 1.
In FIG. 1, a power supply voltage (a) of typically 9 V is supplied to a first stabilized power source circuit 3 and a second stabilized power source circuit 4 from the battery 1 through the power switch 2. A specified reference voltage (b), for example 5 V, is supplied to the printing mechanism 5 from the first stabilized power source circuit 3, while a specified reference voltage (c), for example 5 V, is supplied to a control circuit 6 from the second stabilized power source circuit 4. Moreover, in the conventional circuit shown in FIG. 1, the detecting circuit which detects a drop of the power source voltage (a) supplied from the battery 1 is comprised of a current-limiting resistor 7, a zener diode 8, a semi-fixed resistor 9, a fixed resistor 10 and a comparing circuit 11. A constant voltage (d), for example 2 V, generated by the current limiting resistor 7 and zener diode 8 is directed to one input of the comparing circuit 11, and a voltage (e) divided by the semi-fixed resistor 9 and fixed resistor 10 from the reference voltage (b) output from said first stabilized power source circuit is applied to the other input of the comparing circuit 11. By adjusting the resistance value of the semi-fixed resistor 9, the divided voltage (e) is set to equal the constant voltage (d) when the reference voltage (b) becomes the allowable minimum value 9. Therefore, by comparing said constant voltage (d) with the divided voltage (e) in the comparing circuit 11, it can be determined that the reference voltage (b) has reached the allowable minimum value due to a drop of the power source voltage (a) of the battery 1.
However, this detecting circuit used in the conventional printer includes rather complicated circuit structures and, therefore, not only is the cost increased, but also fine adjustment of semi-fixed resistor is required for increasing detection accuracy. Accordingly, a printer which can be obtained economically by utilizing a simplified detecting circuit has been awaited.